A lot of ink compositions prepared by using thermochromic microcapsule pigments making use of a coloring and decoloring mechanism of a leuco dye have so far been known.
Known are, for example, an aqueous ink composition for reversibly thermochromic writing instruments containing at least water and a reversibly thermochromic microcapsule pigment involving a reversibly thermochromic composition comprising (a) an electron-donative coloring organic compound which is a leuco dye, (b) an electron-accepting compound which is a developer and (c) a reaction medium which is a discoloration temperature controlling agent for determining a temperature causing the color reaction between both described above, wherein an average particle diameter of the reversibly thermochromic microcapsule pigment described above falls in a range of 2.5 to 4.0 μm, and the particles of 2.0 μm or less account for 30% by volume or less based on the whole microcapsule pigment (refer to, for example, patent document 1) and an aqueous ink composition for reversibly thermochromic writing instruments comprising water, a reversibly thermochromic microcapsule pigment involving a reversibly thermochromic composition comprising (a) an electron-donative coloring organic compound which is a leuco dye, (b) an electron-accepting compound which is a developer and (c) a reaction medium which is a discoloration temperature controlling agent for determining a temperature causing the color reaction between both described above, a polymer flocculant, a comb type polymer dispersant having a carboxyl group in a side chain and a water-soluble resin, wherein an average value of maximum outer diameters of the microcapsule pigment described above is 0.5 to 5.0 μm (refer to, for example, patent document 2).
In general, however, the microcapsule pigment described above is decreased in color intensity as an average particle diameter thereof is smaller, which is undesirable. That fact is described in a paragraph 0023 of patent document 1 described above such that it is difficult to exert color developability at high intensity in the microcapsule pigment having an average particle diameter of less than 2.5 μm and that the particles of 2.0 μm or less account for 30% by volume or less, more preferably 10% by volume or less based on the whole microcapsule pigment. Also, it is described in patent document 2 described above that an average value of maximum outer diameters of the microcapsule pigment is 0.5 to 5.0 μm, but observing the respective examples, the microcapsule pigment of 2.3 to 2.5 μm is used. This is attributable to that a capsule thickness in the microcapsule pigment is relatively increased.
On the other hand, when the above microcapsule pigment is used for an aqueous ink ball point pen, the pigment having a small average particle diameter is preferably used in terms of the writing property and storage stability of the ink. However, the existing situation is that inks containing a thermochromic microcapsule pigment and ball point pens which satisfy the various performances have not so far been obtained because of the reasons described above.
Prior Art Documents
Patent Documents
    Patent document 1: Japanese Patent Application Laid-Open No. 2008-280440 (claims, examples and others)    Patent document 2: Japanese Patent Application Laid-Open No. 2009-227954 (claims, examples and others)